JPH03183726A - Production of hot rolled steel plate excellent stretch flange formability - Google Patents

Abstract

PURPOSE:To obtain a hot rolled steel plate excellent in stretch flange formability and tensile strength without deteriorating external appearance characteristics by subjecting a steel in which respective contents of C, Si, Mn, P, S, Al, and Fe are specified to rolling, cooling, and coiling under respectively prescribed conditions. CONSTITUTION:A steel having a composition which consists of, by weight, <=0.02% C, <=0.1% Si, 0.3-3% Mn, <=0.1% P, <=0.005% S, 0.01-0.1% Al, and the balance Fe and in which the relations in 90XC(%)+10XSi(%)+8XMn (%)+88XP(%)>=10 is satisfied is refined. A slab of the above steel is hot-rolled and then finish-rolled at >=(Ar3-30 deg.C). Subsequently, the resulting rolled slab is cooled at >=5 deg.C/sec cooling rate and coiled at <=800 deg.C, by which a structure in which tensile strength is regulated to >=38kgf/mm<2> and pearlite or martensite comprises <=3% and also ferrite comprises >=95% can be formed.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention is mainly aimed at automobile parts to be press-formed.
The present invention relates to a method for producing a hot-rolled steel sheet that has a tensile strength of 10 mm or more and excellent stretch flangeability.

(Conventional technology) In recent years, there has been a strong demand for higher strength and thinner automobile parts to reduce weight, and steel plates have been made to have higher strength, so-called D
Although ual Phase steels and the like have been developed, the actual situation is that these high-strength hot-rolled steel sheets crack in members that undergo severe stretch-flange processing, so the number of applicable members has been limited.

(Problems to be Solved by the Invention) In order to overcome this situation, methods for manufacturing hot-rolled steel sheets with excellent stretch flangeability are disclosed in Japanese Patent Laid-Open No. 60-149730 and Japanese Patent Laid-Open No. 51-44508, respectively. Disclosed. However, the method of adding Si to increase strength as disclosed in JP-A-60-149730 has the problem of poor appearance due to the formation of St scale.

The method disclosed in Japanese Patent Application Laid-Open No. 51-44508 is a manufacturing method for a soft steel sheet of JIS standard 18spHE class, and has a tensile strength of 38, which is the objective of this patent.
It was not intended to be a high-strength steel plate with a strength of kgf/mm" or higher, and it was economically disadvantageous because it required the addition of Cr.

The present invention was made in view of these problems, and provides a method for manufacturing hot rolled steel sheets that achieves both stretch flangeability and strength, which have become important for hot rolled steel sheets for automobile parts, without impairing appearance properties or economical efficiency. provide.

(Means for solving problems) The gist of the present invention is as follows.

(1) In weight%, C: 0.02% or less, Si: less than 0.1% 1M0:0
．． 5-3%, P: 0.1% or less, S + 0.005
% or less, contains Ar2: 0.01 to 0.10%, and satisfies the following formula (1), 90 x C (%) + 10 x Si (%) + 8 x Mn (%
)+88×P (%) k 10・1)1 After forming the steel consisting of the balance Fe and unavoidable impurities into a slab, it is hot rolled and finish rolling is completed at a temperature of Ar5-30°C or higher,
After finish rolling, cool at a rate of 5°C/second or more and heat to 800°C.
The tensile strength is 38 kgf/m or more,
A method for producing a hot-rolled steel sheet with excellent stretch flangeability, characterized in that the structure is 3% or less pearlite or martensite and 95% or more ferrite.

(2) In weight%, C: 0.02% or less, Si: less than 0.1%, Mn: 0
．． 5-3%, P: 0.1% or less, S: 0.005
% or less, 19:0.01 to 0.10%, and Ca
: 0.0005-0.0050%, T i :
0.01-0.06%.

B: Contains one or more of 0.005% or less and satisfies the following formula (1), 90 x C (%) + 10 x Si (%) + 8 x Mn (%) + 88 Xp (%) ≧ 10・・・・・・
...(1) After making the steel consisting of the balance Fe and unavoidable impurities into a slab, it is hot rolled and finished rolling is completed at a temperature of Ar5-30°C or higher, and after finishing the finishing rolling, it is cooled at a rate of 5°C/second or higher. The tensile strength is 38 kgf.
/1) A method for producing a hot-rolled steel sheet with excellent stretch flangeability, which is 1 m" or more in length and has a structure of 3% or less pearlite or martensite and 95% or more ferrite.

(for production) Next, the reasons for limiting each component of the present invention will be explained in detail.

First, factors that influence stretch flangeability will be explained in detail. Stretch flangeability is generally evaluated by a punch hole expansion test, and the fracture mechanism at this time can be broadly divided into void generation, growth, and coalescence, and these occur in stages, leading to cracks penetrating the plate thickness. As a result of detailed research on this mechanism,
The following facts were revealed.

(2) Inclusions tend to be a source of large voids during punching, especially A-type inclusions made of sulfides and the like that extend in the rolling direction, which tend to be a source of voids and deteriorate stretch flangeability. Therefore, it is essential to reduce inclusions to improve stretch flangeability.

■Pearlite or martensite becomes a source of large voids during punching, and at the same time promotes the growth and coalescence of voids, deteriorating stretch flangeability. Therefore, the structure needs to be 3% or less of pearlite or martensite and 95% or more of ferrite.

The ferrite referred to here may be either polygonal or axial.By the way, conventional dual phase steel has a martensitic phase, while other conventional high-strength steel sheets have a pearlite structure. However, both had poor stretch flangeability.

Next, the reason for limiting the chemical components will be explained in detail.

C is one of the most important elements in the present invention. That is, for the purpose of the present invention, it is necessary to keep pearlite or martensite at 3% or less, but C has the greatest effect on the presence of these structures, and if C is 0.02% or less, pearlite Alternatively, a structure can be formed in which martensite is 3% or less and ferrite is 95% or more. Therefore, the upper limit of C was set to 0.02%. Preferably it is 0.01% or less.

Si is a cause of Si scale and deteriorates chemical conversion treatment properties, so the content was set to less than 0.1%.

Mn is an element necessary to ensure tensile strength, and 0.
The content must be 5% or more. The upper limit is due to workability in steel manufacturing.
The rate was set at 3% after comprehensively considering economic efficiency and other factors.

P may be added as a reinforcing element, but since it has the effect of reducing weldability, the upper limit was set at 0.1%.

In the present invention, solid solution strengthening is used to ensure tensile strength, and in order to obtain a tensile strength of substantially 38 kgf/m- or more, 90×C (%) + 1OxSi (%) + 8×Mn (
%)+58xP (%)≧10.

S increases A-based inclusions and deteriorates stretch flangeability, so it needs to be reduced, and the upper limit is set at 0.005%. Preferably it is 0.003% or less.

Aff is necessary as a deoxidizing agent. If it is less than 0.0%, there is no effect. If it exceeds 0.10%, alumina-based inclusions increase, deteriorating the ductility and stretch flangeability of the steel.

Ca may be added to reduce A-based inclusions by controlling the form of inclusions. Addition of o,oos% without a drop has no effect on form control, and addition of more than 0.005% has no effect on form control. The upper limit was set here because not only does it saturate, but it also increases Ca-based inclusions, which causes an adverse effect.

Ti has the effect of reducing solid solution C and solid solution N, improving aging properties, and controlling the form of inclusions. Ti may be added to take advantage of these effects, but 0.01% or more is required to exhibit these effects, so the lower limit should be set at 0.01% or more. The old percentage was used. The upper limit was set at 0.06% in consideration of economic efficiency.

B has the effect of strengthening grain boundaries and may be added.
In order to exhibit this effect, a content of o,ooot% or more is required; however, if it exceeds o,oos%, the effect not only becomes saturated, but may also lead to deterioration of elongation. Addition of B is particularly effective in the case of extremely low C.

Next, the hot rolling conditions will be explained in detail.

First, the melted slab may be hot-rolled after being inserted into a heating furnace and heated, provided that the finish rolling finish temperature is maintained, or it may be hot-rolled directly without being inserted into a heating furnace. In addition, when hot rolling a slab by inserting it into a heating furnace, the heating method may be to heat the molten slab as a hot piece, or - after it has been cooled, it can be rolled from a cold piece. May be heated.

The finish rolling finish temperature is specified as Ar5-30°C or higher. Since the steel of the present invention has a low C content, the finish rolling finish temperature is set at Ar5-30°C or higher.
Even if it is slightly lower than point m, there is little deterioration in workability, but A r
s Below -30°C, the processed structure of ferrite remains strong and the deterioration of workability increases, so the lower limit is set to Arm 3.
The temperature was 0°C. Note that the upper limit of the finish rolling finishing temperature is preferably 1000° C. or less in consideration of rolling workability.

Cooling after completion of finish rolling may be carried out by a normal method, and the cooling rate should be 5°C/sec or more in consideration of prevention of extreme coarsening of ferrite grains and workability from hot rolling to coiling. Since the steel of the present invention has a low C content, there is no concern that martensite will occur even if the cooling rate is a little high.Therefore, it is not necessary to set an upper limit on the cooling rate, but there is no problem if the cooling rate is 300°C/sec or less.

The winding temperature shall be 800°C or less. If the temperature exceeds 800℃, the scale will become extremely thick, and if it is pickled afterwards, the workability will be reduced, and if it is used in the black bark, a large amount of scale will peel off, which will worsen the working environment. The preferred range is 750°C or lower and 50°C or higher.

The steel strip according to the present invention may be cut into plates on a shearing line, and at that time, the shape may be adjusted using a leveler or skin pass rolling, and curls may be corrected.

(Example) Steel having the components shown in Table 1 was melted in a converter and made into a slab by continuous casting.

In Table 1, the steels designated by symbols A-F are within the scope of the present invention, and the steels designated by symbols G-J are outside the scope of the present invention.

For Gm, Mn and the value of formula (1) are below the lower limit, S for H steel is above the upper limit, and C9Si for I steel and J steel is above the upper limit.

Table 2 shows the hot rolling conditions and microstructure/rolling characteristics. In Table 2, the finished plate thickness was 2.9 m+n, and rolling was carried out for 20 to 30 minutes after completion of rolling. After that, apply 0.8% skin pass and make it into a product.
It was subjected to material testing. Note that No. 4 is an example in which the slab was hot rolled immediately after melting without being inserted into the heating furnace. No, 4
In all cases, the slabs were heated in a heating furnace to the respective temperature and then hot rolled. In addition, the structure and its area ratio were determined using an optical microscope.

For the tensile test, a No. 5 test piece according to JIS 22201 was used.

Stretch flangeability was evaluated by the hole expansion ratio in a punched hole expansion test.

The test piece was a square steel plate with a 250 mm+ piece, a punch with a diameter of 20 mm, and a die ([20,0+ 2 x plate thickness x 0.
1] A hole with a diameter d0 (=die diameter) was punched out using a die with a diameter of 1 mm.

For the hole expansion test, the punched hole in the above test piece was expanded using a press testing machine using a 30° conical punch from the side without burrs (opposite the burrs) of the punched hole (at this time, the material was The crack was stopped when it penetrated the plate thickness, and the hole diameter (dl) and the original hole diameter (do
) ratio (d/dot) was taken as the hole expansion ratio.

In Table 2, Nos. 1 to 9 are steels of the invention examples, which had the strength and good hole expansion ratio targeted by the invention, and had no Si scale generation and good surface quality. . In addition, No. 1 has cementite present in the ferrite grain boundaries, but there is substantially no pearlite or martensite, 95% or more is ferrite, and it has a good hole expansion ratio.

No. 10 to 14 are comparative steels. No, l
Like No. 1, O has a structure containing most of ferrite and a small amount of cementite, and has a good hole expansion ratio, but because the Mn content is low and the value of formula (1) is also low, the strength is insufficient. No. 1) had a high S content and a large amount of A-based inclusions, resulting in a low hole expansion ratio. In No. 12, the C content was too high and the pearlite content exceeded 3%, resulting in a low hole expansion ratio. No. 13 is Dual Pl
Since the steel was +ase steel and the martensite content exceeded 3%, a sufficient hole expansion ratio could not be obtained despite the implementation of control of the morphology of inclusions by adding Ca. Nao N.

At 12.13, a cloud pattern due to Si scale was also observed. No. 14 has a finish rolling finish temperature of Ar5-
This is an example in which the temperature was too low, below 30°C, and the ferrite was processed into elongated grains, resulting in poor elongation and a low hole expansion ratio.

(Effects of the Invention) As explained above, according to the manufacturing method of the present invention, a hot-rolled steel plate having excellent stretch flangeability and a bow trap strength of 38 kgf/m+a" or more can be produced with good appearance properties and economic efficiency. This makes it possible to increase the strength of parts that are difficult to stretch-flange, making it easier to reduce the weight of automobile parts, etc., and achieving economic effects such as improving fuel efficiency and saving resources. Vine.

Claims (2)

[Claims] (1) In weight%, C: 0.02% or less, Si: less than 0.1%, Mn: 0.
5-3%, P: 0.1% or less, S: 0.005% or less,
Contains Al: 0.01 to 0.10% and satisfies the following formula (1), 90 x C (%) + 10 x Si (%) + 8 x Mn (%) + 88 x P (%) ≧ 10 ・・・・・・・(1) After making the steel consisting of the balance Fe and unavoidable impurities into a slab, it is hot rolled and finish rolling is completed at a temperature of Ar_3-30℃ or higher, and after finish rolling is completed 5℃/sec. After cooling at a temperature of 800℃ or less, the tensile strength is 38kgf.
/mm^2 or more, pearlite or martensite is 3
A method for producing a hot-rolled steel sheet with excellent stretch flangeability, characterized in that the structure is 95% or more ferrite. (2) In weight%, C: 0.02% or less, Si: less than 0.1%, Mn: 0.
5-3%, P: 0.1% or less, S: 0.005% or less,
Contains Al: 0.01 to 0.10%, and Ca: 0.0
005-0.0050%, Ti: 0.01-0.06%
, B: Contains one or more of 0.005% or less and satisfies the following formula (1), 90 x C (%) + 10 x Si (%) + 8 x Mn (%) + 88 x P (%) )≧10 (1) After making the steel consisting of the remainder Fe and unavoidable impurities into a slab, it is hot rolled and finish rolling is completed at a temperature of Ar_3-30°C or higher, and after the finish rolling is completed. Cooled at 5℃/second or more, rolled up at 800℃ or less, tensile strength 38kgf
/mm^2 or more, pearlite or martensite is 3
A method for producing a hot-rolled steel sheet with excellent stretch flangeability, characterized by having a structure in which ferrite accounts for 95% or less.